Identification of primordial germ cell-like cells as liver metastasis initiating cells in mouse tumour models

Do PGCCs in Solid Tumors Appear Due to Treatment-related Stress or Clonal Expansion of CSCs that Survive Oncotherapy?

Dedifferentiation of epithelial cells during epithelial-mesenchymal transition (EMT) results in circulating tumor cells (CTCs) that are mobilized singly or in clusters in association with blood cells and results in metastasis. However, lineage tracing studies have failed to delineate any role of EMT during metastasis. Research is also focused on polyploid giant cancer cells (PGCCs) in solid tumors which appear in response to oncotherapy-related stress for their role in metastasis. But how to explain PGCCs role in metastatic tumors in treatment-naïve patients? Studies done using mouse models and clinical samples suggest that cancer initiates due to dysfunctions of tissue-resident, pluripotent very small embryonic-like stem cells (VSELs). VSELs are the most primitive and pluripotent stem cells that exist at top of cellular hierarchy in multiple tissues. They are normally quiescent and undergo asymmetrical cell divisions to give rise to two cells of different sizes and fates including smaller cells to self-renew and bigger tissue-specific progenitors. Progenitors undergo symmetrical cell divisions and clonal expansion (rapid proliferation, endoduplication with incomplete cytokinesis) to form giant cells that further breakdown and differentiate into tissue-specific cell types. Oncotherapy destroys actively dividing cells, but CSCs survive. We hypothesize that excessive self-renewal and clonal expansion of cancer stem cells (CSCs, dysfunctional VSELs) result in multinucleated giant cells (PGCCs) that accumulate as further differentiation into tissue-specific cell types is blocked in cancerous conditions. PGCCS are being reported by multiple groups whereas CSCs remain elusive due to small size and low abundance and actually contribute to both cancer initiation and metastasis.

Activation of embryonic/germ cell-like axis links poor outcomes of gliomas

Background

It is unclear which core events drive the malignant progression of gliomas. Earlier studies have revealed that the embryonic stem (ES) cell/early PGC state is associated with tumourigenicity. This study was designed to investigate the role of ES/PGC state in poor outcomes of gliomas.

Methods

Crispr-Cas9 technology, RT–PCR and animal experiments were used to investigate whether PGC-like cell formation play crucial roles in the tumorigenicity of human glioma cells. Bioinformatic analysis was used to address the link between ES/PGC developmental axis and glioma overall outcomes.

Results

Here, our findings showed that germ cell-like cells were present in human gliomas and cultured glioma cells and that the formation of germ cell-like cells was essential for glioma tumours. Bioinformatic analysis showed that the mRNA levels of genes related to embryonic/germ cell development could be detected in most gliomas. Our findings showed that the activation of genes related to reprogramming or the germ cell-like state alone seemed to be insufficient to lead to a malignant prognosis, whereas increased mRNA levels of genes related to the activation of the embryonic/germ cell-like cycle (somatic PGC-EGC-like cycle and somatic parthenogenetic embryo-like cycle) were positively correlated with malignant prognoses and poor clinical outcomes of gliomas. Genes related to the embryonic/germ cell cycle alone or in combination with the WHO grade or 1p19q codeletion status could be used to subdivide gliomas with distinct clinical behaviours.

Conclusion

Together, our findings indicated that a crucial role of germ cell-like cell formation in glioma initiation as well as activation of genes related with the parthenogenetic embryo-like cycle and PGC-EGC-like cycle link to the malignant prognosis and poor outcomes of gliomas, which might provide a novel way to better understand the nature of and develop targeted therapies for gliomas as well as important markers for predicting clinical outcomes in gliomas.

The foundational framework of tumors: Gametogenesis, p53, and cancer

The completion-of-tumor hypothesis involved in the dynamic interplay between the initiating oncogenic event and progression is essential to better recognize the foundational framework of tumors. Here we review and extend the gametogenesis-related hypothesis of tumors, because high embryonic/germ cell traits are common in tumors. The century-old gametogenesis-related hypothesis of tumors postulated that tumors arise from displaced/activated trophoblasts, displaced (lost) germ cells, and the reprogramming/reactivation of gametogenic program in somatic cells. Early primordial germ cells (PGCs), embryonic stem (ES) cells, embryonic germ cells (EGCs), and pre-implantation embryos at the stage from two-cell stage to blastocysts originating from fertilization or parthenogenesis have the potential to develop teratomas/teratocarcinomas. In addition, the teratomas/teratocarcinomas/germ cells occur in gonads and extra-gonads. Undoubtedly, the findings provide strong support for the hypothesis. However, it was thought that these tumor types were an exception rather than verification. In fact, there are extensive similarities between somatic tumor types and embryonic/germ cell development, such as antigens, migration, invasion, and immune escape. It was documented that embryonic/germ cell genes play crucial roles in tumor behaviors, e.g. tumor initiation and metastasis. Of note, embryonic/germ cell-like tumor cells at different developmental stages including PGC and oocyte to the early embryo-like stage were identified in diverse tumor types by our group. These embryonic/germ cell-like cancer cells resemble the natural embryonic/germ cells in morphology, gene expression, the capability of teratoma formation, and the ability to undergo the process of oocyte maturation and parthenogenesis. These embryonic/germ cell-like cancer cells are derived from somatic cells and contribute to tumor formation, metastasis, and drug resistance, establishing asexual meiotic embryonic life cycle. p53 inhibits the reactivation of embryonic/germ cell state in somatic cells and oocyte-like cell maturation. Based on earlier and our recent studies, we propose a novel model to complete the gametogenesis-related hypothesis of tumors, which can be applied to certain somatic tumors. That is, tumors tend to establish a somatic asexual meiotic embryonic cycle through the activation of somatic female gametogenesis and parthenogenesis in somatic tumor cells during the tumor progression, thus passing on corresponding embryonic/germ cell traits leading to the malignant behaviors and enhancing the cells' independence. This concept may be instrumental to better understand the nature and evolution of tumors. We rationalize that targeting the key events of somatic pregnancy is likely a better therapeutic strategy for cancer treatment than directly targeting cell mitotic proliferation, especially for those tumors with p53 inactivation.

To Be or Not to Be a Germ Cell: The Extragonadal Germ Cell Tumor Paradigm

In the human embryo, the genetic program that orchestrates germ cell specification involves the activation of epigenetic and transcriptional mechanisms that make the germline a unique cell population continuously poised between germness and pluripotency. Germ cell tumors, neoplasias originating from fetal or neonatal germ cells, maintain such dichotomy and can adopt either pluripotent features (embryonal carcinomas) or germness features (seminomas) with a wide range of phenotypes in between these histotypes. Here, we review the basic concepts of cell specification, migration and gonadal colonization of human primordial germ cells (hPGCs) highlighting the analogies of transcriptional/epigenetic programs between these two cell types.

Lessons from the Embryo: an Unrejected Transplant and a Benign Tumor

Embryogenesis is regarded the 'miracle of life', yet numerous aspects of this process are not fully understood. As the embryo grows in the mother's womb, immune components, stem cells and microenvironmental cues cooperate among others to promote embryonic development. Evidently, these key players are frequently associated with transplantation failure and tumor growth. While the fields of transplantation and cancer biology do not overlap, both can be viewed from the perspective of an embryo. As an 'unrejected transplant' and a 'benign tumor', lessons from embryonic development may reveal features of transplants and tumors that have been overlooked. Therefore, eavesdropping at these natural complex events during pregnancy may inspire more durable approaches to arrest transplant rejection or cancer progression.